As technology progresses, portable devices tend to integrate more features. For example, portable devices may include features associated with personal digital assistants (PDAs), cellular telephones, wireless internet access devices, global positioning system (GPS) receivers, and the like. Such portable devices may support one or more wireless communications protocols, such as third (3G), fourth (4G), or later generation cellular telephone protocols, GPS protocols, wireless fidelity (Wi-Fi) protocols, Bluetooth®, and the like. Some of these protocols may have tight channel spacing that require narrow channel bandwidths and tight restrictions on transmissions of spurious RF signals outside a channel's bandwidth. Often, to meet channel bandwidth requirements and to meet spurious RF emissions requirements, an RF transmitter may need to meet stringent linearity and noise requirements. An RF power amplifier in an RF transmitter may be a major source of non-linearity; therefore, there is a need for an RF power amplifier that meets stringent linearity requirements.
Most portable devices are battery powered and, since battery life is inversely related to power consumption, need to consume as little power as possible to maximize battery life. The RF power amplifier in a wireless portable device may consume a significant portion of the power used by the portable device. One way to reduce power consumption in an RF power amplifier is to improve its efficiency. Some communications protocols, such as those using wideband code division multiple access (WCDMA), may require polar modulation of an RF signal. Polar modulation includes an amplitude modulation (AM) component and a phase modulation (PM) component. To maximize efficiency in an amplitude-modulated or polar-modulated RF power amplifier, an envelope power supply input to the RF power amplifier may be provided by an AM power supply, which may include a switching power supply that tracks, at least to some extent, the amplitude of an amplitude-modulated or polar-modulated RF signal to the RF power amplifier.
Linearity of the RF power amplifier may be degraded when using a constant power supply due to modulation of the gain of the RF power amplifier by the RF envelope, which is associated with AM. Additionally, the AM power supply must provide adequate headroom for proper operation of the RF power amplifier. In some systems, such as 4G or later communications protocols, the bandwidth of an AM signal used to modulate the AM power supply may be about 12 megahertz or more; therefore, for proper tracking, the bandwidth of the AM power supply may need to be about 12 megahertz or more. Such an AM power supply may include a switching power supply circuit for high efficiency and a linear power supply circuit to meet bandwidth requirements; however, linear power supplies are typically less efficient than switching power supplies. Therefore, a 12 megahertz AM power supply may be less efficient than an AM power supply with less bandwidth. Thus, there is a need for an AM power supply having wide bandwidth and high efficiency that allows RF power amplifier circuitry to meet linearity requirements.